A system for examining Discontinuous DNA synthesis in vitro on cellophane discs has been developed. Using this system, we will attempt to elucidate: a) The mechanism of initiation of discontinuously synthesized DNA intermediates. b) The possibility of specific switchover from RNA primer to deoxynucleotides during discontinuous DNA synthesis. c) The role of different factors in DNA replication, particularly the DNA-G gene product. d) The role of ATP in DNA replication. e) The mechanism of coupling of synthesis on the two complementary DNA strands, and the possibility of uncoupling this synthesis in vitro. In addition, we have two more general objectives: 1) The development of a new system for assaying insoluble components of the DNA replication fork. 2) The development of an in vitro system for mammalian DNA synthesis which is free of intact nuclei.